Sorry to say this, but Butterfree isn't a Purple Pokemon :/ According to the Pokedex, he is white! You can find lists of Pokemon by colour on various websites, notably Bulbapedia. The fully evolved Gen 1 purple Pokemon are; Arbok, Nidoking, Golbat, Venomoth, Muk, Cloyster, Gengar, Weezing, Starmie, Aerodactyl and Ditto. Any of these Pokemon would be fine!